Traffic signal lights consisting of hundreds of light emitting diodes (LEDs) have recently been developed. These LED traffic signal lights are intended to replace the conventional incandescent light bulbs in ordinary traffic signals. Some of these devices can be mounted in the same housing that is currently used for the incandescent bulbs; and some designs also incorporate the same type of electrical connector, so that these LED traffic signal lights can be used as plug in replacements for incandescent bulbs.
LED traffic signal lights can be designed to produce, with normal line voltage, the same light intensity as incandescent bulbs that are currently used, and to have comparable performance characteristics for different viewing angles. In addition, these LED traffic signal lights have significant advantages over incandescent bulbs. First, most LED traffic lights achieve a dramatic decrease in energy consumption. Such an LED traffic light can use as little as 15% as much energy as an incandescent bulb, although the energy savings for different designs can vary significantly. This energy conservation can save municipalities a substantial amount of money and, not incidentally, help to protect the environment and energy resources. A second major advantage of these LED traffic lights is their reliability. Municipalities typically replace every incandescent bulb in all of their traffic signals every year. In stark contrast, an LED traffic light normally has a useful life of approximately 15 years. There are also less obvious advantages of the LED traffic signal lights over the incandescent bulbs. By way of example, because of the lower energy consumption, the required electrical current capacity and cost for the wiring in new traffic signals is lower.
Although the advantages of LED traffic lights can be readily demonstrated, the different electrical characteristics of the LED over the incandescent bulb has substantially inhibited use of the LED light. Thus, one advantage of the incandescent bulb is that it can generate adequate light intensity to control traffic at a highway intersection despite a substantial drop in the input supply voltage. A typical conventional traffic signal will normally provide 120 volts of input power to an incandescent bulb. When the input supply voltage drops to about 75% of its normal value, an incandescent bulb with red filter can still generate approximately 50% of its normal intensity. Such voltage drops (often referred to as "brownouts") often occur in summer, when the electrical energy resources are overloaded.
In contrast, the intensity of light generated by a typical LED traffic light can decrease to as little as 3% of its normal intensity when the input supply voltage drops to 75% of its normal value. Several LED traffic lights in the prior art simply rectify an input voltage and place this voltage across serial strings of LEDs so that the voltage across each LED drops as the input supply voltage drops. Because of the electrical characteristics of the LEDs used in these LED traffic lights, the intensity of light generated by each LED decreases dramatically as its voltage drop decreases. As a result, Such traffic lights appear very dim when the input supply voltage drops substantially. This results in very dangerous situations, especially in crowded urban or suburban areas, and especially in conditions of reduced visibility. Whenever the power supply to a given area is disrupted, for whatever reason, so that the supply voltage drops to a brownout condition (approximately 92 volts alternating current (AC)), these LED traffic lights will not produce sufficient light to effectively control traffic.
One prior art approach that has been used in an attempt to solve this problem involves providing a direct current (DC) power supply for each LED traffic signal light, where the power supply can operate over a wide range of input voltages. This approach supplies an approximately constant voltage to the LEDs despite variations in the traffic signal supply voltage. A second approach that has been used involves connecting a resistor and a number of LEDs in series. The resistor limits the current flowing through the LEDs when the input voltage is at its normal value. But when the input voltage drops, the resistor helps to maintain the voltage differential across the LEDs by absorbing a portion of the voltage drop.